In this chapter, the author views trust as the confidence in the association of a stable network execution to the efficient distribution of multimedia products in the final user. A network is stable under a greedy protocol (or a composition of protocols) if, for any adversary of injection rate less than 1, the number of packets in the network remains bounded at all times. The author focuses on a basic adversarial model for packet arrival and path determination for which the time-averaged arrival rate of packets requiring a single edge is no more than 1. Within this framework, the author studies the property of stability under various compositions of contention-resolution protocols and different packet trajectories trying to characterize this property in terms of network topologies. Furthermore, the author enhances the adversary allowing the monitoring of network link capacities/slowdowns. Within this context, the author shows how the stability properties of network topologies change when network link slowdowns/capacities can change dynamically. Interestingly, his results indicate that a composition of protocols leads to worst stability behaviour than having a single unstable protocol for contention-resolution. This suggests that the potential for instability incurred by the composition of protocols may be worse than that of some single protocol. Consequently, this study could help on the design and maintainance of trustworthy heterogeneous multimedia systems.
TopIntroduction
Nowadays, there is a necessity for the development of fast and reliable large-scale communication platforms for the distribution of commercial multimedia products. Within this context, it is important the presence of a trust relationship among all the entities that cooperate (vendors, consumers, network infrastructure). However, in a distributed system or a network, the entities involved in a trust relationship may have no direct knowledge of each other so there is a need for mechanisms to support trust among them. A trust relationship has specific properties (Grandison & Sloman, 2000): (i) It is not absolute. (ii) It may not be symmetric. (iii) It should not be transitive. However, some trust scenarios do exhibit transitivity. (iv) It is associated with a level of trust. Some entities may be trusted more than others with respect to performing an action. We should take into account these properties in order to design trustworthy multimedia networks.
An important problem of multimedia networks is their vulnerability to denial of service attacks that attempt to degrade network performance (Levine & Kessler, 2002). Such attacks can jeopardise the user's confidence in (or specifically distrusts) the standard of quality of service provided by the network. Therefore, in distributed environments the required service trust is directly related to the reliability or integrity properties of the network infrastructure. Especially, in e-commerce and e-banking network applications, the customer or vendor trusts the network to support mechanisms that will ensure that all the transactions will be handled fast and reliable.
The performance and reliability properties of multimedia networks are closely related to stability. Stability requires that the number of packets in the network remains bounded at all times against any adversary under a single contention-resolution protocol or a composition of protocols. (By composition of contention-resolution protocols, we mean the simultaneous use of different such protocols at different queues of the network.). Adversarial attacks that can lead a network to instability can be seen as a type of denial of service attacks since their purpose is to flood the network (or a subnetwork) with packets whose sole purpose is to overload the local system in order to hamper (or prevent) legitimate users from having access to the system. If a network is proven to be stable its users are ensured that the network services are trusted against adversarial attacks. Therefore, the users can develop a trust relationship with the network infrastructure.
Studying the stability behaviour of a network is not an easy task. The complexity of this study increases taking into account the heterogeneity that characterises modern large-scale communication platforms, such as the Internet. Heterogeneity comes around in many different flavours. For example, different traffic sources over the Internet (due to varying mechanisms for supporting different service qualities) result in a heterogeneous mix of traffic traces. Moreover, although, conceptually, the Internet uses a unified set of protocols, in practice each protocol has been implemented with widely varying features (Clark, 1988; Floyd & Paxson, 2001). Thus, heterogeneity is a crucial feature that makes it difficult to model, verify and analyze the behaviour of such large-scale multimedia networks. As the Internet evolves into a ubiquitous communication infrastructure that supports multiple protocols running on different network hosts, its dependability in the presence of various adversarial attacks becomes critical. These attacks can degrade system performance and lead to service disruption. Therefore, the study of performance properties of heterogeneous systems which suffer from adversarial attacks can help to certify the trust properties of such systems. Thus, the researchers will not only be informed of a better design for establishing and maintaining a trustworthy heterogeneous multimedia system, but they will also be assisted in the understanding of the concept of trust in a heterogeneous multimedia environment.